1. Field of the Invention
The present invention relates to an information recording medium such as an optical disk for use in an information recording/reproducing apparatus for optically recording and reproducing information, and, more particularly, to an information recording medium with an increased recording capacity and improved reliability of recorded information.
Recently, information recording/reproducing apparatuses, such as optical disk apparatuses, have been developed which record information on an information recording medium such as a write once or erasable optical disk or reproduce information recorded on the recording medium. According to such information recording/reproducing apparatuses, an optical head which emits light is moved by a linear motor in the radial direction of an optical disk so as to face a recording or reproducing position on the disk. When the optical head reaches the target position, it emits light to record information on the optical disk or reproduce information through photoelectric conversion of the reflected light from the optical disk.
A typical recording system for use in such optical disk apparatuses is the CAV (Constant Angular Velocity) system which keeps the number of rotations of the optical disk constant. This CAV system has merits of ensuring stable data recording and reproducing and shortening the required access time. According to this system, however, the number of clocks for data recording and reproducing or the frequency of data modulation and demodulation is constant irrespective of the location of target data on an optical disk, at the innermost track or at the outermost track. Accordingly, an optical disk for use in an optical disk apparatus employing this CAV system has header information preformatted thereon using a header data transfer clock of a constant frequency. The CAV system therefore has a demerit of reducing the data recording density as the target position on the optical disk for data recording or reproducing approaches the outermost track.
There is another recording system for use in optical disk apparatuses, which uses the CLV (Constant Linear Velocity) system. This system provides a constant linear velocity and thus a constant recording density on an optical disk by reducing the angular velocity of the optical disk or the number of rotations per unit time in accordance with movement of the position of an optical head facing the optical disk from the inner track side toward the outer track side while keeping constant the number of clocks for data recording and reproducing or the frequency of data modulation and demodulation irrespective of the location of target data on an optical disk, at the innermost track or at the outermost track. An optical disk for use in an optical disk apparatus employing this CLV system has header information preformatted thereon at a constant linear density. This CLV system can increase the recording density on an optical disk and can therefore increase the recording capacity per optical disk. Due to the necessity to alter the number of rotations of an optical disk, the CLV system requires the time to wait for the rotational speed to reach the desired level. This undesirably results in a longer time for accessing to the target track for data recording or reproducing.
As a solution to the above shortcomings of the CAV and CLV systems, a constant linear density system has been developed which keeps the linear density on an optical disk constant. This system provides a constant recording density on an optical disk by increasing the frequency of the data transfer clock in proportional to movement of the position of an optical head facing the optical disk or the recording position toward the outer track side of the optical disk while keeping the rotational speed of the disk constant.
Since the constant linear density system should increase the frequency of the data transfer clock in accordance with a change in the recording position toward the outermost track, however, it should inevitably satisfy severe data recording conditions on the side of outer tracks on an optical disk. In addition, due to the existence of information recording media of various recording systems, there is a demand for a method for properly identifying the types.